Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have advantages such as high energy density, excellent long-term reliability and the like, and therefore they have been put into practical use in notebook-type personal computers and mobile phones. In recent years, the performance of electronic device has been improved and its use in electric vehicles and the like have been advanced, and thus further improvement of battery characteristics such as capacity, energy density, safety and the like are strongly desired.
For example, there have been made various studies on heat-resistant separators for improving the safety of lithium ion secondary batteries. For example, Patent Document 1 discloses a separator for a battery using a porous film composed of an aromatic polyamide represented by a particular formula, in which aromatic rings having para orientation occupy 90 mol % or more of all aromatic rings, the coefficient of static friction μs of 0.3 to 1.8, and Mw/Mn which is the ratio of the weight average molecular weight Mw to the number average molecular weight Mn of the aromatic polyamide is in the range of 1.3≤Mw/Mn≤4.5. In addition, Patent Document 2 discloses that by the use of a separator of laminated porous film having the laminate of a heat-resistant layer containing a heat-resistant resin and a shutdown layer containing a thermoplastic resin, the thermal film-breakdown of the secondary battery can be prevented.
On the other hand, in order to improve the energy density of a lithium ion secondary battery, it is effective to increase the electrode density. However, in the electrode having a large electrode density, since the porosity in the electrode becomes small, there has been a problem that impregnation of the electrolyte solution into the electrode is insufficient.
In order to solve such problems, a method of increasing the impregnation time from when the electrolyte solution is injected to when initial charge is performed, a method of adding small vibration and periodic vibration at the time of injecting the electrolyte solution, and a method of heating a battery container after injecting the solution, for example such as a method of controlling an outer wall temperature of a battery can to be 30° C. or higher and 50° C. or lower (Patent Document 3) and the like have been proposed.